The present invention relates to a vehicle brake controller that adjusts braking force applied to wheels provided on a vehicle and to a vehicle brake control method.
When the vehicle is suddenly braked while running, the center of gravity of the vehicle may move forward. In particular, when the vehicle is suddenly braked while running on a high-μ road at a low speed, the center of gravity of the vehicle moves significantly forward, and a rear-wheel lift-up may occur, in which the vertical load of the rear wheels with respect to the road surface is reduced while the vertical load of the front wheels with respect to the road surface is increased. Since the occurrence of the rear-wheel lift-up results in the unstable behavior of the vehicle, limit control for limiting increase in the braking force applied to the front wheels is preferably executed if there is a possibility of occurrence of the rear-wheel lift-up. When the limit control is executed, forward movement of the center of gravity of the vehicle is reduced, and the rear-wheel lift-up becomes less likely to occur.
When performing such limit control, it is necessary to estimate the amount of movement of the center of gravity of the vehicle, and determine whether there is a possibility of occurrence of the rear-wheel lift-up. For example, Japanese Laid-Open Patent Publication No. 2007-203810 discloses a method for calculating the pitch angle speed, which is the rate of change of the pitch angle of a vehicle body (hereinafter, referred to as a first method). The first method includes acquiring the rate of change of the up-down direction acceleration of the front portion of the vehicle body and the rate of change of the up-down direction acceleration of the rear portion of the vehicle body, and computing the pitch angle speed based on the two rates of change of the up-down direction acceleration. If the computed pitch angle speed is greater than a predetermined threshold value, it is determined that there is a possibility of occurrence of the rear-wheel lift-up due to the significant movement of the center of gravity of the vehicle.
However, in the first method, two up-down direction acceleration sensors need to be provided in the vehicle to detect the up-down direction acceleration of the front portion and the rear portion of the vehicle body. This increases the costs of the vehicle. Thus, a second method has been proposed that estimates whether there is a possibility of occurrence of the rear-wheel lift-up without using detection signals from the up-down direction acceleration sensors.
In the second method, elapsed time is measured from when deceleration of the vehicle associated with the braking operation by the driver is detected. If front-back direction deceleration of the vehicle exceeds a start determination value before the elapsed time reaches a predetermined start determination time period, it is determined that there is a possibility of occurrence of the rear-wheel lift-up due to the significant forward movement of the center of gravity of the vehicle. The front-back direction deceleration used in such determination process is a value based on a detection signal from a front-back direction acceleration sensor provided in the vehicle.
Since the front-back direction deceleration is a value acquired by subjecting a detection signal from the front-back direction acceleration sensor to a process such as known filtering, the front-back direction deceleration changes after a delay from the actual deceleration of the vehicle. Also, since the front-back direction acceleration sensor used in such control is not necessarily always arranged at the position of the center of gravity of the vehicle, a response delay corresponding to the distance between the arrangement position and the position of the center of gravity of the vehicle may occur in the detection signal from the front-back direction acceleration sensor. Therefore, the start determination value is preferably set to a relatively small value to avoid delay in starting of the limit control.
However, disturbance such as vibration according to variation in the road surface reaction force that the vehicle wheels receive from the road and vibration associated with variation of the gear range of the transmission of the vehicle are likely to be superimposed on the detection signal from the front-back direction acceleration sensor. Therefore, when the start determination value is set to a relatively small value, when the front-back direction deceleration is temporarily increased by the influence of the disturbance, the starting condition of the limit control is satisfied even though sudden braking is not performed. Thus, the limit control may be unnecessarily started. In this case, as compared to a case where the limit control is not executed, the braking force of the entire vehicle is decreased by the amount corresponding to the decrease in the braking force applied to the front wheels.
The start determination value is preferably set to a relatively large value to reduce unnecessary executions of the limit control described above. However, in this case, even if the vehicle is actually suddenly braked, the starting condition of the limit control is not easily satisfied, and the starting of the limit control may be delayed.